Age hardening stainless steel

ABSTRACT

CORROSION AND HEAT RESISTANCE, WORKABLITITY AND TOUGHNESS OF AGE-HARDENABLE FE-MO ALLOYS ARE IMPROVED BY THE ADDITION OF CR, NI AND CO. THE AGE HARDENABILITY IS REINFORCED BY THE ADDITION OF W, BE TI AND SI. RESISTANCE TO NON-OXIDIZING ACIDS IS IMPROVED BY ADDITION OF CU.

United States Patent fice us. 01. 75-128 T 6 Claims ABSTRACT OF THE DISCLOSURE Corrosion and heat resistance, workability and toughness of age-hardenable [Fe-Mo alloys are improved by the addition of Cr, Ni and Co. The age hardenability is reinforced by the addition of W, Be, Ti and Si. Resistance to non-oxidizing acids is improved by addition of Cu.

DETAILED DESCRIPTION OF INVENTION The present invention relates to analloy with a body centered cubic structure having high strength and high resistance to corrosion in which resistance to corrosion, workability, toughness and resistance to heat are increased by adding Cr, Ni and Co to Fe-Mo ageing alloy and ageing property is reinforced by adding W, Be and Ti. The object of the invention is to provide a relatively inexpensive alloy with high strength as construction material and spring material which require high resistance to corrosion.

It is well known that age hardening appears by adding Mo to Fe by more than 15%. Such alloys, however, can be used only as cast alloy because it is impossible to apply such work as rolling and drawing, besides its resistance to corrosion is insuificient.

On the other hand, our invention is charatcerized by the fact that resistance to corrosion is greatly increased by adding Cr to Fe-Mo alloy, and the amount of Mo required for age hardening is reduced and sufiicient workability can be obtained.

Further it is found that toughness, resistance to heat, resistance to corrosion are increased without spoiling the age hardening of the material by adding Co and Ni, singly or complexed, to Fe-Mo-Cr alloy, and that age hardening property is greatly increased by adding W, Be and Ti.

Composition of this alloy will be disclosed in the following.

(1) Cr6 to 22% Cr stabilizes Fe material, and increases resistance to corrosion of the material. In the alloy according to the invention, too, Cr is added to increase resistance to corrosion, the amount of Mo of Fe-Mo age hardening alloy is decreased by adding Cr. Thus workability for cold rolling and drawing etc. is improved without spoiling age hardenability. With Cr less than 6% in content resistance to corrosion is insufficient, with Cr over 22% in content the material tends to harden and spoils the workability.

(2) Mo+W4 to 13% ggs) Element M in Fe-Mo alloy produces age hardening effect. The alloy according to our invention improves not only age hardening but also resistanceto corrosion against non-oxidized acid. Element W in Fe material, like as Mo,

3,677,744 Patented July 18, 1972 shows age hardening property. The effect of W on age hardening property is weaker than that of Mo. However, it shows greater reinforcement for solid solution than Mo, so it strengthens the matrix of this alloy. Too much addition, however, makes the material brittle and deteriorates its workability. After examined each alloy from three points, i.e. age hardenability, solid solution reinforcement, brittleness of the material, it was found that Mo+W should be 4 to 13%, W being up to of the total amount added.

(3) Co-Ni Percent Co only 0.5 to 12. Ni only From 0.5 up to 5. Co and Ni complexed 0.5 to 12. 1

1 Ni less than 4.5

Co and Ni greatly increase toughness of the material and improve workability of rolling and drawing. These elements improve resistance to corrosion. Especially Ni improves resistance to corrosion at normal temperature and Co improves acid proofness at high temperature. The amounts added should be as described above. When the amounts added is less than the lower limit, there is little effect and when the amount exceeds the upper limit, the material tends to brittle and workability deteriorates. Ni with more than 5% added, tends to produce 7 phase, therefore too much addition should be avoided.

(4) Ti-Be Percent Ti only 0.1 to4 Be only 0.05 to2 Ti and Be complexed 0.05 to 3.5

Ti and Be in Fe material stabilize ferrite. With the lowering of the temperature, its solubility decreases and age hardening elfect appears. The alloy according to the invention has body centered, ferrite crystal construction. By means of these elements strength is greatly increased without spoiling its hardenable property. This great effect is due to the fact that treatment condition for hard ageing of Fe-Mo and that of Fe-Be and Fe-Ti are similar, and that the precipitation compounds which contribute to hardening have completely diiferent phases and not influence with each other. Therefore Ti and Be added together do not spoil the hardening eifect. When the amount added is less than the lower limit, harden-ability deteriorates and when the amount added of Ti and Be, singly or mixed together is over the upper limit, the material becomes brittle and working becomes diflicult.

(5) Si--less than 2% Si deoxidizes melting metal and promotes ageing of Fe-Mo alloy. With Si more than 0.5% in content age hardening appears. However when the amount of Si exceeds 2%, the material tends to be brittle.

(6) Cu--less than 3% Nb reacts with C to form stable carbide and improves resistance to corrosion. Particularly in thi alloy, Ti, M0 and Cr easily reacts with C and deteriorates not only 3 resistance to corrosion but also age hardenability. By adding Nb, carbide of Nb stable at higher temperature is formed, which prevents the reaction with these elements. However, when Nb is added by more than 0.3%,

Table 1 Table 2 shows hardness, tensile strength and elongation of these alloys. From this result, it can be seen that addition of Ni or Co has great influence on toughness of the 1t g g g g f ifigii g gg ggg than material. Alloys from Nos-. 17 to 3-3 have higher elongaan e s o tion than those from Nos. 1 to 16 which do not include Ni and Co. Alloys including such age hardening elements 8 Cless than 0.1 as T1 or Be, show higher hardness in melting than those C easily forms carbide with Mo, Ti and and which do not include them. These alloys when melting deteriorates resistance to corrosion and age hardenability. have the sijlme strength 17-4 PH 17 7 PH on C should be avoided as much as possible, but it is enevitmarket Whlch are known as age hardemng. Stamless steelably intermingled during melting operation. Considering TABLE 2.STBENGTH OF MELTING MATERIAL the above fact, is added in the present alloy. Nb of Hard, Tamils I less than 0.1% m content is sufiicient. n ess strength gation Referring now to the embodiment of the invention, v) (kg'lmm'a) (percent) Table 1 shows a typical composition of the alloy accord- 202 102 8 ing to the present invention. For comparison, two kinds 2 3 L of age hardening stainless steel on the market and that $3 g {g of 1 8-8 stainless steel are also shown. 246 98 13 In Table 1, all alloys Nos. 1 to 33 were melted in vacg 32 uum by high frequency induction furnace wherein degree 302 103 4 10 of vacuum in melting was maintained at 3 to 8x10- 2:3 :3? mm. Hg. Such melting material as electrolytic pure iron, 298 95 12 pure Cr, Ferro-Mo, Ferro-W, pure Co, pure Ni, Ferro-Ti, Q? i% a Ferro-Be, Ferro-Nb, Ferro-Si, electrolytic pure copper 306 99 12 were used. They were melted in such a manner that elecl3? trolyticpure iron, pure Cr, Ferro-Mo, Ferro-W, pure Co, 313 103 M pure N1 were charged in high pure alumina crucible and 3g? 3% g the furnace was made vacuous, then the melting operation 34. 111 2 was started. Added Fe-Si after the material was melted g3? 8g down in the crucible. The temperature was set to the 300 104 27 value 100 to 200 C. above the melting temperature when 2%; i8 22 the melting metal lbecame clean. Threw in quickly Cu, 300 101 20 Ferro-Nb, Ferro-Ti and Ferro-Be in this order. One or 35 2%; *3? 33 two minutes after Ferro-Be was melted, ingot was cast. 281 00 30 The temperature was measured by optical pyrometer. if: g Surface of these ingots were made smooth and band 208 108 28 material was formed by hot forging, hot and cold rolling, i.e. hot rolling at 1050 to 900" C. and heated up to 1150 fig 8? to 1200 C. and quenched by water to be used for test 153 00 pieces for strength and resistance to corrosion.

TABLE 1 Cr Mo W 00 Ni Ti B0 S1 Cu Nb 0 Fe Alloys according to the invention:

1 8.03 11.12 0.48 0.10 0.02 Balance. 2.- 10.12 0.08 0.20 2.50 0.02 Do. 3.- 12.05 0.11 0.31 2.40 0.21 0 03 Do. 4.. 14.05 5.80 2.33 2.01 0.02 Do. 5-- 15.00 8.01 0.03 1.01 0 12 0.02 Do. 0.. 15.10 9.08 1.02 1.04 0 11 002 Do. 7-- 20.05 10.05 0.03 Do. 8.- 20.12 7.20 0.28 2.05 0.02 Do. 0-- 5.33 7. 21 3.58 2.74 0.08 0.25 1.04 0.11 0 02 Do. 10. 7.82 11.31 1.35 0.37 0.10 0.10 0.02 Do. 11. 8.45 5.08 0.23 0.35 0.42 2.01 0.02 Do. 12. 11.27 8.47 04 0.22 0.13 0 02 Do. 13- 12.58 5.27 1.57 2 85 Do. 14. 14.75 8.45 0.70 Do. 15. 15.12 0.22 52 0.34 1 D0. 15- 18.38 0.05 0.48 Do. 17. 0.30 7.14 0. 34 0. Do. 18--- 8.36 10.02 0.42 Do. 10--- 11.01 8.10 0.22 Do. 20 15.11 0.01 0.01 D0. 21--- 10.42 8.53 0.38 Do. 22 15.03 0.03 1.08 Do. 23 8.55 0. 57 0.42 Do. 24. 10.34 5.04 0.04 Do. 25 15.01 8.80 1.02 Do. 25--- 18.42 5.03 7. s 0.00 Do. 27.-. 7.21 11.01 0.3 0.23 Do. 28 14.90 7.28 0.34 0 Do. 20-.- 7.15 10.80 5. 2. 1.23 0. Do.

10.77 8.11 .20 3. 0.41 0 12 .02 Do. 0.02 1. 0.00 0.88 0.10 0.02 Do. 5.03 4.35 1.12 4.35 1.89 0.10 0.14 0.35 -.003 Do. 4.50 4.72 4.25 2. 0.43 1.10 0.02 Do.

Table 3 shows hardness and tensile strength of 50% cold rolled material .and those when this alloy is aged. Age hardening treatment was performed both in the alloy according to the invention and the material compared under such a condition that the maximum strength was obtained in the preliminary experiment. All heat treatment was performed in the vaccumfurnace in order to avoid oxidation. From the results shown in Table 3, it can be seen that before age hardening treatment there is little difference between the alloys according to the invention and the alloys compared, 17-4 PH, 17-7 PH and 18-8 stainless steel, but that there appears great difference between them after aged, which shows that the ageing property of the alloys according to the invention is excellent.

After age hardening treatment test pieces in Table 3 were immersed in various corrosion liquids for ten days and its degree of corrosion and discoloration was examined. The results are shown in Table 4.

Alloys according tothe invention Tensile e n -l condition 2 Hardness Hardness Hv) 570 C. 7H 216 681 95' Sam; as above 203 694 Artificial sea water and artificial sweat in Table 4 are TABLE 3 p Age hardening 2O sweat and sea water made artificially by mixing chem- %ro11edmate1ia1 mate ial icals, the components of which are similar to those of Anoys' Tensile Tensile natural sweat and sea water. Their compositions are as according strength Hardstrength Hardf ll w to the 7 (kg./ ness Ageing (kg./ ness invention mmfi) (Hv) condition mm?) (Hv) v 1; H 122 42 0x711 205 720 Component of artificial sweat: Percent 2 g8 22 as 195 Nacl 0.648 to 0.987 4; .126 9 238 Na s 0.006 to 0.025 g--- fig 23% (NH Co 0.086 to 0.173 7: 132 497 NH OH 0.010 to 0.018 gggg $33 Saccahrose 0.006 to 0.022 16 148 723 Lactic acid 0.034 to 0.107 11 150 723 82 Component of artificial sea water: 14 144 741 Nacl 2.30 to 2.35 i8 iii $83 Mgcl 0.45 to 0.49 17 150 758 Na2SO4 t0 i3 i2? 3%? CaCl 0.10 to 0.12 39 g; ;g; Kcl 0.063 to 0.066 22 134 694 NaHCO 0.018 to 0.020 23 148 KBr 0.0080 to 0.0095

TABLE 4 Artificial sea 10% 10% 5% 10% lactic 20% water Nacl Hcl H280 NH4cl NH OH N323 acid NaZCO;

O A O A O 8 A O A 8 A A A g A A A O O A 8 8 8 8 8 8 8 O O O g O A A A O O O a 2 2 a a a a a O 2 8 2 O 8 8 o o A 8 8 O A A g O O O 0 O A A A O 8 (0 O A O A O O A O A 8 O O 5 O O O O O A O A O 8 A 8 8 8 8 E 8 8 O O 8 A O A g 2 O O O O A O A O a A a 6 a a a 8 O O 0 o 0 g o A X X X O O O A 25 A A X X 0 O O O A A A No'rE.=No discoloration or corrosion; A=Discoloration and slight corrosion; O=Slight discoloration;

From Table 4 it can be seen that the alloys according to the invention have higher corrosion resistance than 17-4 PH and 17-7 PH which are known as age hardening stainless'steel. It shows similar resistance to corrosion with that of l88 stainles steel which has high work hardenability.

. As described above, the alloy according to the invention has excellent age hardenability and corrosion resistance and is suitableas the construction members for machine and spring members, in which high corrosion resistance is required.

Whe we claim is:

1. Age-hardenable, corrosion-resistant stainless steel, consisting essentially of 640% Cr, 412% of Mo+W, the ratio of W to Mo+W being up to 0.6, 0.5-% Ni, up to 2% of Si, up to 3% of .Cu, carbon is present in an amount up to 0.1% Nb in quantity suflicient only to form carbide with substantially all of the carbon present, but in any case less than 0.3% and a metal selected from the group consisting of Ti, Be and mixtures thereof, where the Ti content in the. absence of. Be is in the range of 0.l-4%, the Be content in the absence of Ti is in the range of ODS-2%, and the total Ti+Be content when introduced as said mixture is in the range 0.053.5%, the balance being Fe. 7

2. Age-hardenable, corrosionqesistant stainless steel consisting essentially of about 6.4% Cr, about 7.1% Mo, about"3.5% W, about 2.3% Ni, about 1.4% Ti, about 0.3% Si, about 1.1% Cu, about 0.02% C, the remainder being Fe.

3. Age-hardenable, corrosion-resistant stainless steel consisting essentially of about 8.4% Cr, about. 10.9% Mo, about 4.6% Ni, about 1.4% Be, about 0.4% Si, about 0.1% Nb, and about 0.02% C, the balance being Fe.

4. Age-hardenable, corrosion-resistant stainless steel consisting essentially of about 11.9% Cr, about 8.1% Mo, about 1.2% W, about 0.8% Ni, about 0.2% Ti, about 0.4% Be, about 0.2% Si, about 1.0% .011, about 0.1% Nb, and about 0.02% C, the balance" being. Fe. 5. Age-hardenable, corrosion-resistant" stainless steel consisting essentially of about 19.4%"Cr, about "816% Mo, about 1.4% W, about 3.2% Ni',-about 2.0% Ti, about 0.1% Be, about 0.4% Si, about 0.6%:- Cu, and about 0.02% C, the balance being Fe. 1

6. Stainless steel as defined in claiml, whereinsaid Mo content is 4-12%, said W content is 07.2%, 'said Si content is O.22%, said Cu content is O.'5-3.0%,.and said C content is 00.1%.. 1

- References Cited UNITED STATES PATENTS r 1,587,614 6/1926 Strauss 128 W 2,185,996 1/ 1940 Hatfield 7512"8 W 2,747,989 5/ 1956 Kirkby 75 i1 28 W 3,152,934 10/1964 Lula 75- 128 W 3,154,412 10/1964 Kasak 7 5 1 28 121] 2,403,141 7/1946 Touceda 75-126. G

2,881,236 4/1959 Evrgets 75-1Z6 C 2,905,577 9/1959 Harris 75-126 C HYLAND BIZOT, Primary Examiner US. Cl. X.R. 

